In October, Twitter co-founder Evan Williams handed Costolo -- a onetime improv performer -- the top spot, a tacit acknowledgment that finding a way to make Twitter profitable has become the top priority. The microblogging service has grown to 160 million registered users -- 65% of the Fortune 100 had active accounts by this summer -- though monetizing that base has been tough. This year the company hired Adam Bain from News Corp. and Ali Rowghani from Pixar to divine new revenue streams. Perhaps the company's new ad platform, Promoted Trends, will help.

Executives from Silicon Valley to Hollywood to Wall Street admires his savvy persistence - and his company's cool culture. The secret to the Netflix CEO's success? He never stops looking over his shoulder. More

Michael Dell was 21 when he first appeared in Fortune. Bill Gates made the cover when he was 30. Here's a look back at some of today's biggest business leaders when they were climbing the corporate ladder. More